I don't know why Android 2.xx on phones lets you do it, but after all you aren't likely to run out room just with apps.
At least some Android 2.x devices (phones and cheap tablets) have a very small space
allocated to internal storage. My Archos 101 paperweight has a VERY small space for
internal storage. Add a few apps, maybe a game or two, and you're out of space. My
Motorola Bravo phone has most of its apps on the main storage and the biggest ones
on the sdcard, and while they would likely all fit on the main storage area, it wouldn't
have a lot of extra space for more. So yeah, at least on the older devices with smaller
storage allocations, or rather, at least SOME of the older devices.....there's a very
real reason.
Anyways, I was mostly just curious. Thanks.